Phosphorescent organic light emitting diodes (OLEDs) are under intensive investigation because of their potential of achieving improved device brightness and performances. In contrast to the fluorescent emission, the electrophosphorescence of heavy metal complexes are easily generated from both singlet and triplet excited states and, thus, the internal quantum efficiency can reach a theoretical level of unity, rather than the 25% inherent upper limit imposed by the formation of singlet excitons for the respective fluorescent counterparts. Thus, a great deal of effort has been spent on the second and third-row transition metal complexes, for developing highly efficient phosphors that can emit all three primary colors. Despite of the elegant research on both red and green phosphors, there are only scatter reports on the room temperature blue phosphors. The best known example is one Ir(III) complex named as FIrpic in the following, which has proved to be an excellent dopant for sky-blue phosphorescent OLEDs. Further improvements were made by substituting picolinate with other ancillary ligands such as pyridyl azolate ligand to afford derivative complexes FIrpyz or FIrtaz shown in the following. These modifications have produced a hypsochromic shift of ˜10 nm versus the emission of FIrpic; however, significant lowering of Q.Y. was noted in some cases, which have hampered the fabrication of the true-blue phosphorescent OLEDs.

In theory, one has to consider some critical design in achieving the higher efficient blue phosphorescence. One possibility is to increase the MLCT contribution in the lowest lying triplet manifold. The direct involvement of non-bonding, metal dπ orbital enhances the coupling of the orbital angular momentums with the electron spin, such that the T1→S0 transition would have a large First-order spin-orbit coupling term, resulting in a drastic decrease of radiative lifetime and hence a possibility of increasing emission Q.Y. For probing such possibility, this group have prepared two isomeric, blue-emitting heteroleptic iridium (III) complexes, namely complexes (dfppy)Ir(fppz)2 (complex 1 in the following) and (dfppy)Ir(fppz)2 (complex 2 in the following). The contribution of MLCT character, which serves as a key factor in spin-orbit coupling enhancement, is calculated to be 27% and 17% for (dfppy)Ir(fppz)2 (complex 1) and (dfppy)Ir(fppz)2 (complex 2), respectively, according to the DFT calculation. As such, the respective theoretical analysis revealed an increase the dπ contribution in (dfppy)Ir(fppz)2 (complex 1) versus that of complex 2, rendering a larger First-order spin-orbit coupling term and hence shortening radiative lifetime as well as increasing the emission Q.Y., which are consistent with the experimental observations.

Conversely, care has taken to avoid the enhanced radiationless deactivation pathways due to the purposed enlargement of the emission band gap. One familiar deactivation pathway lies in the population to the metal-centered dd excited states, which may cause the weakness of the metal-ligand bond, resulting in a shallow potential energy surface. In an extreme case, the shallow dd potential surface may intercept with other surfaces of states and greatly channel into the radiationless deactivation. This process, however, may be minor for the third-row transition metal elements due to their strong coordination strength that far pushes up the dσ* orbitals.
As for the third consideration, upon increasing the energy gap toward true-blue, it becomes facile for the lowest lying excited state, a state perhaps consisting of both ππ* intraligand charge transfer (ILCT) and metal-to-ligand charge transfer (MLCT) in character, to mix with a thermally accessible ligand-to-ligand charge transfer (LLCT) character. Owing to its largely charge-separated character and hence partially forbidden transition probability (versus the ground state), mixing with the LLCT excited state may eventually increase the radiative lifetime, hence reduce the corresponding Q.Y. if similar deactivation mechanisms are operative. Theoretically, the participation of LLCT excited states can be suppressed by employing the facial arranged homoleptic complexes, for which the excitation is equally spread among the degenerate states of multiple chromophores. The delocalization of the electron density would not only stabilize its molecular framework but also reduce the radiationless deactivation simply due to the resulting steeper potential energy surfaces. Moreover, for the unsymmetrical meridional isomer, the three chelate ligands are located at the distinctive environment, the non-degenerated nature of these chelate molecular orbitals would then facilitate the LLCT character and giving the poor emission Q.Y. at room temperature. Such a hypothesis was confirmed by a recent investigation on the photophysical behavior of the related homoleptic complex mer-[Ir(fppz)3] (complex 3 in the above), for which an unprecedented dual phosphorescence, i.e. a blue (P1) and a green (P2) bands deriving from the ILCT and LLCT excited states, are observed at room temperature. It is notable that the ILCT and LLCT excited states of mer-[Ir(fppz)3] are nearly orthogonal to each other and possessing mainly the ligand ππ* character together with a small extent (˜10%) and an enhanced (20%) MLCT character, respectively. Thus, the ILCT to LLCT energy transfer, which takes place at room temperature with small barrier possibly due to certain large-amplitude motions, would also induce the rapidly quenching of the higher energy, blue phosphorescence.